Various color liquid crystal projectors have previously been proposed in which different light beams are optically superimposed so as to be projected onto a screen to form a full color image. Each beam is modulated by a different liquid crystal light valve array having picture information input thereto that corresponds to a color component of the image. After the different component light beams are combined, the combined beam is projected onto a screen so that the full-color image can be viewed.
Recently, it has become important for color liquid crystal projectors to have sufficient brightness so as to be viewed in a room under conditions of partial lighting. This requirement, combined with the trend toward smaller pixel size and higher resolution of LCD light valves, has made it imperative that the projection lens of a color liquid crystal projector have high resolution and form a bright image. Also, a very long back focus is required in such a projection lens so that a color-synthesizing optical element can be inserted in the space between the projection lens and the liquid crystal light valve arrays.
Furthermore, it is desirable that the projection lens be telecentric on its reducing side. Retrofocus lenses, as used for wide-angle lenses of single-lens reflex cameras, have been known as wide-angle lenses that have a long back focus. However, retrofocus lenses used as wide-angle lenses of conventional single-lens reflex cameras have a steep emitting angle and thus are not telecentric on the side of the light source. As a result, when such retrofocus lenses are used, a problem arises in that irregular colors occur on the viewing screen. Therefore, conventional retrofocus lenses cannot be used as projection lenses for color liquid crystal video projectors.
In conventional focusing methods for projection lenses, in order to maintain a stationary image surface during focusing, a method wherein the entire lens is integrally moved and a method wherein plural lens groups are moved independently are known. However, in such conventional focusing methods there are problems in that the structure for mounting and moving the movable components becomes complicated.
As a technique for solving problems as discussed above, applicant previously disclosed a retrofocus lens in Japanese Laid Open Patent Application H10-260346. Techniques described in this patent can solve the problems as described above. On the other hand, it is difficult to realize a lens having an F.sub.NO in the range of 3.5-4.0. Nevertheless, applicant invented such a projection lens in order to produce a brighter image, as disclosed in Japanese Patent Application No. H11-351693. The overall length of the projection lens disclosed in this application is about 9.2 units in the case where the focal distance of the projection lens is 1 unit. Thus, as compared to similar conventional techniques, although providing a compact lens, there remains a demand for further miniaturization.